Various methods of producing vinyl-siloxane block copolymers have been discussed in the literature including anionic polymerization, condensation polymerization and thermal free radical polymerization. However, the methods discussed in the art are complex and are limited as to the selection of monomers which can be successfully polymerized. Anionic polymerization, for example, is limited to the use of monomers such as nonpolar vinyl and diene compounds. Such limitations of anionic polymerization are discussed in an article by Noshay and McGrath, entitled "Block Copolymers", Academic Press, New York, 1977, pp 157, 278, 410. Limitations in the production of vinyl-siloxane block copolymers are also discussed in an article by Saam, Ward and Fearon, entitled "Polystyrene-polydimethylsiloxane Multiblock Copolymers", Advanc. Chem. Ser., No. 129, 239-47 (1973). The Saam article discusses the platinum-catalyzed polycondensation of .alpha., .omega.-hydrogen terminated polydimethyl siloxane and .alpha., .omega.-vinyl terminated polystyrene utilizing anionic polymerization for the synthesis of the .alpha., .omega.-vinyl terminated polystyrene and the drawbacks associated therewith.
We believe that block copolymers of siloxane and vinyl monomers which can be easily synthesized and tailored can combine the properties of two dissimilar polymers thereby providing unique materials which potentially have broad utility in a wide range of areas.
The present invention provides novel siloxane iniferter compounds and vinyl-siloxane block copolymers made by utilization of the novel siloxane iniferter compounds as a means of promoting, controlling and terminating polymerization of the block copolymers.
The term "iniferter" refers to a chemical compound that has a combined function of being a free radical initiator, transfer agent, and terminator, the term "iniferter" being a word formed by the underlined portions of the terms identifying these functions. This term and its use in a production of block copolymers is well known, particularly because of the work of Takayuki Otsu of the Department of Applied Chemistry, Osaka City University, Osaka, Japan. This work is discussed, for example, in an article by Otsu et al entitled "Living Radical Polymerizations in Homogeneous Solution by Using Organic Sulfides as Photoiniferters", Polymer Bulletin, 7, 45-50 (1982), an article by Otsu et al entitled "Living Mono- and Biradical Polymerizations in Homogeneous System Synthesis of AB and ABA Type Block Copolymers", Polymer Bulletin, 11, 135-142 (1984), and in European Patent No. 0286376, published October 12, 1988. Despite the rather detailed description of making other block copolymers according to such disclosures, there is no disclosure of the novel siloxane iniferter compounds or the vinyl-siloxane block copolymers made therewith herein claimed.
Copending U.S. application Ser. No. 07/212,594, Ali et al., filed June 28, 1988, (assigned to the assignee of the present case) discloses the use of iniferter technology in the preparation of acrylic block copolymers having the requisite physical properties making them suitable for use in pressure-sensitive adhesive compositions. The control of the polymerization permits tailoring of the reinforced acrylic block copolymer to provide a balance of adhesion, cohesion, stretchiness and elasticity to make a successful pressure-sensitive adhesive. Copending U.S. application Ser. No. 07/212,594, filed June 28, 1988, Ali et al, does not disclose siloxane iniferter compounds or the use of such iniferter compounds in the synthesis of vinyl-siloxane block copolymers.
Copending U.S. application Ser. No. 07/212,593, filed June 28, 1988, Andrus Jr. et al., (also assigned to the assignee of the present case) discloses the use of iniferter technology in the preparation of acrylic block copolymers which can be tailored to provide optical clarity and resistance to oxidative and photochemical degradation. The acrylic block copolymers disclosed in copending U.S. application Ser. No. 07/212,593 are employed to make shaped articles, sheet materials, and the like. Copending U.S. application Ser. No. 07/212,593 also does not disclose siloxane iniferter compounds or the use of such iniferter compounds in the synthesis of vinyl-siloxane block copolymers.
Polysiloxane grafted copolymers useful as release coatings prepared by a non-iniferter method that do not require a curing step have been described (U.S. Pat. No. 4,728,571) wherein controlled and predictable release is achieved through variation in the number and the length of the polysiloxane grafts.
Copending U.S. application Ser. No. 07/278,283, filed Nov. 30, 1988, Mertens et al., (also assigned to the assignee of the present case) discloses release coatings for pressure sensitive adhesive tape comprising copolymers of free radically polymerizable vinyl monomer, polar monomer in a sufficient amount to impart a hydrated T.sub.g of -15.degree. C. to 35.degree. C. and a difference of at least 20.degree. C. between the hydrated and actual T.sub.g s, and siloxane-based polymer of a type and in an amount that will impart a satisfactory release value. The release coatings are capable of being written effectively with water and oil based pen inks.
Various methods of preparing block copolymers of silicone and vinyl monomers have been described in the art.
Crivello, U.S. Pat. Nos. 4,584,356; 4,677,169; and 4,689,289, teaches the thermal preparation of vinyl-siloxane block copolymers. Crivello teaches a thermal method involving the use of a macromolecular siloxane initiator (U.S. Pat. No. 4,584,356) and describes utility as E-beam resists (U.S. Pat. No. 4,677,169) and positive or negative resists (U.S. Pat. No. 4,689,289). Crivello does not disclose the use of iniferter technology in the preparation of vinyl-siloxane block copolymers. In addition, Crivello's thermal preparation method can only be used in the synthesis of block copolymers having simple AB or ABA architecture and not in the preparation of block copolymers having more complicated architecture such as CAB,CABAC, etc.
Eichenauer, et al describes condensation of either a semitelechelic hydroxy-terminal vinyl polymeric segment with an acetoxy-terminal siloxane (DE 3,606,984) or a semitelechelic carboxy-terminal vinyl polymeric segment with an aminopropyl-terminal siloxane (DE 3,606,983). Japanese laid-open applications 63-57642 and 63-57644 describe the use of peroxy ester terminated siloxanes as macroinitiators for thermal preparation of vinyl-siloxane block copolymers. H. Inoue, et al, J. Appl. Poly. Sci, 35, 2039 (1988) describes properties of poly(methyl methacrylate)/siloxane block copolymers obtained from thermal polymerization using an azo-containing siloxaneamide macroinitiator.
Inoue, Ueda and Nagai; Journal of Polymer Science Part A, 26, 1077-1092 (1988) is another reference which discusses the thermal preparation of block copolymers. Inoue teaches the thermal preparation of silicone-vinyl block copolymers by radical polymerization of vinyl monomers such as methylmethacrylate, styrene, etc. in the presence of polysiloxane azobiscyanopentaamides which serve as macroazoinitiators. Inoue et al, like Crivello, does not teach the use of iniferter technology in the preparation of vinyl-siloxane block copolymers.
These methods, while useful, involve reactions that are not easily controlled. Since the reactions are not easily controlled, the copolymers formed thereby cannot be easily tailored.
The article entitled "Block Copolymers" by Noshay and McGrath, Academic Press, New York, 1977, pp 156-162, "Block Copolymers--Overview and Critical Survey" which is hereby incorporated by reference discusses phase separation as it applies to silicone-vinyl block copolymers. Noshay does not teach the preparation of vinyl-siloxane block copolymers by use of iniferter technology. Rather, Noshay teaches the preparation of vinyl-siloxane block copolymers by use of anionic polymerization and condensation polymerization methods.
A need therefore exists for a convenient general route of synthesis of vinyl-siloxane block copolymers which permits the tailoring of the block copolymers so that a wide spectrum of physical properties can be introduced. A need also exists for such a method wherein polymeric architecture beyond simple AB and ABA architecture can be obtained. Despite the rather detailed description of making block copolymers according to such disclosures, there is no disclosure of the use of photoiniferter technology in the preparation of the vinyl-siloxane block copolymers herein claimed or the novel siloxane iniferter compounds used in their preparation.
We have found a novel siloxane iniferter which can be used as a means of promoting, controlling and terminating polymerization of a vinyl-siloxane block copolymer.
The siloxane iniferters useful in preparing vinyl-siloxane block copolymers are "macro" iniferters, as opposed to the iniferters of Otsu; Ali et al, U.S. application Ser. No. 07/212,594; and Andrus Jr., et al., U.S. application Ser. No. 07/212,593, which are low molecular weight iniferters which do not contain a polymerized silicone segment or any silicon for that matter. It is not apparent from the above references that a polymerized silicone or siloxane segment could be contained in an iniferter. Both the preparation and function of the "macro" siloxane iniferters useful in preparing vinyl-siloxane copolymers differ from the iniferters described in the references above.
The selection of the endblocker which can cooperate with the siloxane midblock segment to form the "macro" siloxane iniferter useful in preparing tailormade vinyl siloxane block copolymer is very critical and must be made in a way which will not diminish the effectiveness of the siloxane segment. The preparation of such a "macro" iniferter including the determination of useful endblocker in its preparation is not suggested by any of the above references.
The polymerized siloxane segment introduced into the "backbone" of the vinyl-siloxane copolymer by the photoiniferter polymerization technique is a midblock component. The endblocker, which caps the siloxane segment, is selected such that it reacts with a soft nucleophile in order to form a "macro" siloxane iniferter compound. The bond between the photoiniferter and endblocker is broken upon exposure to ultraviolet radiation resulting in an initiator free radical and a terminator free radical. The initiator free radical is capable of polymerizing free radically polymerizable vinyl monomer in order to yield vinyl siloxane copolymer.
With respect to Ser. No. 07/212,594, Ali et al. and Ser. No. 07/12,593, Andrus Jr., et al., both the midblocks and endblocks comprise polymerized acrylic monomers. It is not apparent that a polymerized siloxane segment could be included in an iniferter and polymer formed therewith. Otsu also does not teach "macro" siloxane iniferters or the preparation of vinyl-siloxane copolymers.
By using photoiniferter polymerization technqiues, it is possible to obtain more complicated polymer architectures than are available according to the thermal method of Crivello. The thermally prepared block copolymers of Crivello cannot possess architecture beyond AB and ABA.
In addition, photoiniferter polymerization reactions are much more efficient than thermal polymerization methods in terms of providing cleaner block copolymers which are free from unwanted homopolymers. Moreover, photoiniferter polymerization methods can be used to form block copolymers ranging from very low to very high molecular weights. The photoinifierter method permits the tailoring of the siloxane midblock as well as the endblocks to satisfy customer needs which is not possible using Crivello's thermal method.